Complex fertilizer and method of producing same



Dec. 13, 1955 1 E. ANDRES ETAL COMPLEX FERTILIZER AND METHOD OF PRODUCING SAME Filed July 30. 1952 INVENTORS LOUIS E. ANDRES BERNARD J.QUANQU1N BY AA/wm/ mFdLmOIL zUOd ATTO R N EY fertilizer.

United States Patent O COMPLEX FERTILIZER AND NIETHOD OF PRODUCING SAME Louis E. Andres, Saint Gratien, and Bernard J. Quanquin, Grand-Couronne, France, assignors to Potasse & Engrais Chimiques, Paris, France, a societe anonyme of France Application July 30, 1952, Serial No. 301,781

Claims priority, application France August 14, 1951 12 Claims. (Cl. 71-39) This invention relates generally to fertilizers and more particularly to a method for producing complex fertilizers.

In the manufacture of fertilizers it has been common practice to mix simple fertilizers in their commercial form. For example, nitrogen is introduced by using ammonium nitrate, ammonium sulphate, Chilean nitrate, cyanamid; and phosphoric acid is introduced by using superphosphate, dicalcium phosphate, basic slags, or am. monium phosphates. Potash is added generally in the form of chloride, sylvanite or potassium sulphate. In brief, this type of fertilizer is produced by making a physical mixture of those ingredients and is known as a cornpound fertilizer.

Also known is the type of fertilizer called a complex ln producing this kind of fertilizer, instead of starting with simple fertilizer compounds already manufactured and then combined as a physical mixture, the process uses untreated or partially treated raw materials: nitric acid, raw phosphate, sulphuric acid, gaseous ammonia, and, if desired, potassium salts. These are treated in a series of operations which finally result in producing a fertilizer containing the plant elements chemically combined.

ln manufacturing complex fertilizers, one of the first steps consists in reacting the raw phosphate rock with au acid to produce phosphoric acid after which the reaction product is treated with ammonia. One of the desired objectives is to convert the phosphate into a form which is soluble so that the phosphorus is readily available to the plants when the fertilizer is placed in the soil. It is, of course, known that phosphate in tricalcium form in a fertilizer is undesirable because according to the tests applied to fertilizer it is insoluble in water and ammonium citrate and in this form the phosphate is not available to the plants. The phosphate in dicalcium form is (according to tests applied to fertilizer) insoluble in water but soluble in ammonium citrate and is a desirable form, since in this form the phosphate is available to the plants. The phosphate in monocalcium form is soluble in water and, of course, the phosphorus in this form is available to plants. However, the monocalcium form is not desirable in a complex fertilizer because it has been found that the presence of phosphate in Water soluble form causes the fertilizer to be unstable and fertilizers containing the phosphate in water soluble form do not stand up well and do not remain stable when stored for ex- Y tended periods of time. Furthermore, the presence of the phosphate in monocalcium or water soluble form gives rise to the hazards of spontaneous combustion or explosion. Consequently, it is highly desirable that the phosphate in the fertilizer be in the dicalcium form because in this form it is not soluble in water but is soluble in ammonium citrate and is thus available to the plants when it is placed in the soil and the tricalcium form should be avoided because in this form the phosphate is not available to the plants, and the monocalcium form should be avoided for the reasons mentioned above.

r"ice It is an object of this invention to provide a process of producing a complex fertilizer which does not contain the phosphate in the monocalcium form and in which all of the phosphorus (calculated as P205) is in a form soluble in ammonium citrate. It is a further object of the invention to eliminate in the final product the tricalcium phosphate and monocalcium phosphate so that it contains substantially all of the phosphate in the dicalcium form, which is soluble in ammonium citrate but insoluble in water. Also, according to the process of the invention, the presence of calcium nitrate is avoided, it being known that the calcium nitrate because of its affinity for water causes the fertilizer to cake.

According to the process of the invention, rock phosphate is treated with nitric acid, then with ammonia, followed by treatment with ammonia and sulphuric acid in controlled amounts and nally with ammonia, these treatments being carried out within critical limits of control as will be described in further detail hereinafter. The rock phosphate is rst attacked with nitric acid, this reaction producing phosphoric acid. To this reaction product is added ammonia until the reaction mass or slurry has a pI-l of about 3. Then ammonia and sulphuric acid are added in controlled relative amounts in such manner that the pH is gradually raised to about pH 5. Then ammonia alone is added to raise the pH to the neighborhood of 5.5 to 6.5. These steps are carried out under conditions which insure intimate mixing ofthe reactants and under' conditions whichprevent the temperature from rising substantially above 100 C. or going substantially below C. and it is particularly significant to note that as increments of ammonia and/or sulphuric acid are added to the mass as the process proceeds, the pH should be raised gradually in comparable increments and as the process proceeds the pH should not be allowed to fall back asl increments of ammonia and/or sulphuric acid are added but the pH should be gradually raised until the final pH in the neighborhood of 5.5 to 6.5 is reached.

By adding and reacting the ammonia which is preferably in gaseous form and the sulphuric acid as set forth above, all of the phosphoric acid produced from the phosphate rock is transformed into bicalcium phosphate and the calcium nitrate which may be formed is transformed into calcium sulphate, thereby eliminating from the nished product unwanted hygroscopic calcium nitrate.

Control of the temperature of the reacting masses between 90 C. and 100 C. is important. The reaction is exothermic and if the temperature is allowed to go higher than C. an intense evaporation results and this causes the slurry to thicken undesirably and this makes it diicult to distribute the gaseous ammonia uniformly throughout the reacting mass and unless the ammonia is uniformly distributed the phosphoric acid in certain parts of the slurry will produce unwanted tricalcium phosphate which is insoluble in ammonium citrate. lf the reactions are carried out at a temperature substantially below 90 C. the conditions of hydration of calcium sulphate and bicalcium phosphate are so modified that the slurry would get unduly thick and thus prevent a good absorption of the ammonia.

The amount of sulphuric acid ultimately to be added may be calculated on the basis of the analysis of the raw phosphate rock. For this purpose the phosphate rock may be considered on the basis of its phosphorus content in terms of P205 and its calcium content in terms of CaO. For example, tricalcium phosphate which may be represented by the formula Ca3(PO4)2 may also be regarded as being composed of two oxides: P205, (CaO)3. The raw rock phosphate may, and frequently does, contain an amount of calcium in excess of that required to combine with the phosphate to form tricalcium phosphate. That is, in tricalcium phosphate represented by the formula (CaO)3P2O5, the percentage percentage of P205 is 45.8%.

, lation the Vformula for the rock phosphate may be regarded as (Ca).P205, where x represents the numerical value of the mols of CaO present for each mol of P205. If, for example, the percentage of CaO is higher than mentioned above so that there is present 3.6 mols of CaO for each mol of P205, this may be represented by the formula P205(Ca0)3.6. The amount of sulphuric acid to be added, according to the process of the invention, will be the equivalent .amount which will react with A(x-Z) Ca0. Thus, according to the above explanation, if the rock phosphate is such as to be represented by the formula P205(Ca0)3 s, the amount of sulphuric acid to be added will be 1.6 mols of H2804 for each mol of P205 in the rock phosphate. The amount of ammonia to be added will be that required to raise the pH gradually as the sulphuric acid is added as described hereinafter.

I n the iinal stage of the process when all of the sulphuric acid has been added and the reacting mass of slurry has reached a pH of about 5, by adding b oth ,ammonia and sulphuric acid, then ammonia only is added until the `pI-I is in the neighborhood .of 5.5 lto 6.5. Thatis, the slurry is tested during the final stages of the reaction to determine whether there is still present any phosphate in water soluble form. If water soluble monocalcium phosphate is found to be present, sufficient ammonia is added to convert this into dicalcium phosphate, and when suihcient ammonia has been added as a final step to eliminate all phosphate in the water soluble .monocalicum the rate of introducing the ammonia should be such that the pH rises only gradually and at a substantially uniform rate. Meantime the temperature is controlled by circulating cooling water through the jacket of the jacketed reacting vessel so that vthe temperature of the slurry is maintained between 90 C. and 100 C. After the slurry has reached pH 4 -or thereabout, it willV be found that the relative quantities `of ammonia and sulphuric acid to obtain a comparable increment of increase in pH will vary ,Y

from .the ratio OfammOna and sulphuric `acid introduced in the earlier stages of the process, bearing in mind Ithat these reactants are introduced at a rate and :insuch :pro-

portion as will cause a gradual and usubstantially uniform rate of rise Vin the pH. In this instance, .when 4the slurry has Vreached pH 4 it will have taken 1.32 additional l and .33 additional pound of -sulphuric acid. Having.

y reached a pH 5,- vthere is then added tothe slurry only form, the pH will ordinarily be between 5.5 and 6.5, this variation depending upon variations encountered in dif# ferent batches of raw phosphate rock.

The slurry thus obtained is then dried and subsequently granulated, producing a binary fertilizer (containing N ry after the additions of ammonia and sulphuric acid haveV ybeen completed and this will produce a complete fertilizer which contains the three plant food elements (N, P205,

lKzO) in a complex fertilizer in which the -plant food elements are chemically combined instead of being only a physical mixture of various fertilizer compounds.

The process `of the invention may be carried ont as a batch process or as a continuous or semi-continuous '1 process.

As an example of carrying out a batch treatment on a small scale, 9.2 pounds of ground rock ,phosphate [containing 3'4% P205 and containing 3.6 mols of calcium (as CaO) per mol of .P205 and ,regarded Ias being represented by the formula P205(Ca0) 3:sl may -be reacted with 1.6.5 pounds of 52% nitric acid in a water jacketed vessel to convert all the .P205 to water soluble form. Cooling water is circulated in the jacket to control the temperature in the reacting mass of slurry, `and the vessel '-1,

should be provided with an agitator or 'stirring 4means to insure adequate mixing of the reactants during .the process.

Upon ycompletion of the attack on the phosphate rock by the nitric acid, gaseous ammonia is introduced yinto the slurry until pH 3 is reached. ln this instance .46 pound of YNH3 -will have been added. Then ammonia gas and sulphuric acid (9,8%) Yare simultaneously vfed into the slurry which isY .constantly agitated to insure intimate mixing. In this -early stage ofthe Yprocess the ammonia is introduced at a Vrate which corresponds to about half the weight of Vthe sulphuric, acid (98%,). But in any event the relative .amounts and rate of introduction of the ammonia and sulphuric acid is such that the pH of the slurry gradually rises but the amount of sulphnric acid and its rate -of introduction should not .at any time be such as to permit the pH'to 'fall `bae`k `and ammonia to convert any monocalcium phosphate present to the dicalcium form. In this instance 13 pound of ammonia was required, bringing the pH of the slurry to 5.8.

vThe slurry when dried and granulated will producefafertilizer product analyzing as follows: 16.5% of nitrogen of which v8.25% is in the N03 form and 8.25% is in the NH4 form; a total of 14% lphosphoric acid (P205) of which 13.85% is soluble in ammonium citrate; 0.15%

insoluble in ammonium citrate, and no rwater soluble monocaleium phosphate.

In practising the `invention on a large commercial scale, it has been found to be more expedient to carry out the processV as a Acontinuous one and Vthis may be done by utilizing a plurality of water jacketed reaction vessels connected Vin series as Aindicated in the accompanying ilow diagram shown :in Fig. l, wherein 20 `represents a reaction `cham-ber in which ground rock phosphate is re acted with nitric acid and 21 to 27 represent a series of water jacketed reaction Lvessels veach representing a stage in the process as it proceeds and 28 represents a drying apparatus.

`In a typical run, ground Irock phosphate [containing 34% of P205and 3.6 mols of CaO per mol of 'P205 and regarded as represented by vformula P205(Ca0)3.s] was hour and attacked by nitric acid (52%) introduced atV the rate of 7500 kilos per hour. The products of the reaction of the phosphate rock and nitric acid were continuously delivered in series through seven water cooled reaction vessels each having a capacity of one and eighttenths cubic Ameters and each equipped with agitators to insure a thorough mixing of the reactants; The reaction product from chamber 20 was continuously delivered to the rst stage or vessel 21 wherein ammonia gas was introduced at the rate of 212 kilos per hour but no sulphuric acid. The temperature of the mass was maintainedk at 90 C. and the pH was 3; The slurry from the trst stage was passed to the second stage or vessel 22 wherein ammonia at the rate vof 200 kilos per hour and sulphuric acid (98%) at the rate of 390 Akilos per hour were simultaneously added. The temperature was maintained at 1.00 C. and the Islurry reached pH `3.3. The slurry from 'the second stage `was passed to the third stage K or vessel '23 wherein ammonia at the rate of 20.0 kilos per hour and sulphun'c acid (98%) Iat the rate of 39,0 kilos per hour were simultaneously added. The temperature was maintained at 100 C. and the slurry reachedA pH 3.5. The slurry was then passed to the fourth stage or vessel 24 v wherein Aammonia at the rate of 200.ki1os per hour and sulphuric acid attherate of 390,1ri1osper hour were simultaneonslv added. (Ine temperature was at armadio 100"V C. and the slurry reached pH 4.0. The slurry was then passed to the fth stage or vessel 25 wherein ammonia at the rate of 106 kilos per hour and sulphuric acid (98%) at the rate of 250 kilos per hour were simultaneously added. The temperature was maintained at 100 C. and the slurry reached pH 4.5. The slurry was then passed to the sixth stage or vessel 26 wherein ammonia at the rate of 85 kilos per hour and sulphuric acid (98%) at the rate of 150 kilos per hour were simultaneously added. The temperature was maintained at 100 C. and the slurry reached pH 5.0. The slurry was then passed to the seventh stage or vessel 27 wherein ammonia at the rate of 57 kilos per hour was added but no sulphuric acid. The temperature was maintained at 95 C. and the slurry reached pH 5.8. The slurry from the seventh stage was passed through drying apparatus 28 and granulated, producing ten metric tons of binary complex fertilizer per hour.

The foregoing procedure for treating the slurry after the phosphate rock has been attacked by the nitric acid in chamber 20 may be set forth in tabular form as follows:

The slurry delivered from the seventh stage, after drying, produced ten metric tons of fertilizer product having the following analysis:

Nitrate nitrogen (N03) 8.25% N. Ammoniacal nitrogen (NH4) 8.25% N. Phosphoric acid (P205) total 14.00% P205. Phosphoric acid soluble in ammonium citrate 13.85% P205.v Phosphoric acid insoluble in ammonium citrate 0.15% P205.

Although the process as Carried out on a large commercial scale has been illustrated as having been divided into seven stages in series, it will be understood that this is primarily for purposes of illustrating one manner of practising the method of the invention. It will be understood that a greater or less number of separate stages may be employed. The significant point is that the process is carried out in such fashion that the ammonia and/or sulphuric acid are added in increments and in ratio such that as the process goes forward, after the initial stage of adjusting the pH of the slurry to about 3, the reactants are added in such proportion and rate that the pH of the slurry is caused to rise gradually in small increments to the last stage and prevented from falling lower than it had been in any previous stage and in the last stage ammonia only is added to the extent required to convert any phosphate existing in the monocalcium form to the bicalcium form. v

If it is desired to produce a ternary fertilizer (containing N, P205, K20) the desired quantity of a potassium salt such as potassium chloride (KCl) may be added to the slurry delivered from the final stage, that is, stage 7 as indicated in the flow sheet and intimately intermixed after which the slurry containing the added potassium may be dried and granulated to produce a ternaryv fertilizer which is a complete and stable fertilizer which can be stored, as can the binary fertilizer above described, for extended periods of time without disintegration or deterioration.

For example, in the continuous process above described, if 2750 kilos of potassium chloride (60% K20) is added Phosphoric acid soluble in water to the slurry after all of the sulphuric acid and ammonia. has been added, and then intimately mixed and the mixture then dried, there is produced 12.750 metric tons of ternary fertilizer which analyzes as follows: 6.4% N as N03; 6.4% N as NH4; 11% P205 of which 10.9% is soluble in ammonium citrate; 0.1% insoluble in ammonium citrate; no water soluble P205; and 12.9% watersoluble potassium calculated as B120.

The fertilizer obtained by the method of the invention, having no monocalcium phosphate present which is the water soluble form, can be stored indenitely in bulk without risk of decomposition or alteration and there is no n'sk of spontaneous combustion as is the case with fertilizers which contain water soluble monocalcium phosphate. The final product is substantially free of tn'calcium phosphate and the phosphate is present asbicalcium phosphate. And the process avoids the presence in the fertilizer of calcium nitrate which is hygroscopic and therefore undesirable.

The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalent of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of invention claimed.

What is claimed is:

1. A method of producing a complex fertilizer from water insoluble phosphate rock containing phosphorusy and calcium which when calculated as P205 and CaO may be represented by the formula P205(Ca0)2: where x represents the numerical value of mols of Ca0 to each mol of P205 in said rock which method comprises reacting said rock phosphate with nitric acid in suflicient amount to form a slurry in which all the phosphate is converted to phosphoric acid form which is water soluble, then adding sufcient ammonia to said slurry to adjust its pH to about 3, then gradually adding to the resulting slurry sulphuric acid in an amount equivalent to x-Z mols of H2S04 for each mol of P205 present in the slurry and simultaneously while adding said sulphuric acid also adding ammonia gradually in an amount and at a rate and in a ratio to the sulphuric acid to cause the pH of the slurry gradually to rise to a pH of about 5, and then adding suicient ammonia alone to the slurry to raise the pH of the slurry to the neighborhood of 5.5 to 6.5 thereby to convert any existing monocalcium phosphate to dicalcium phosphate and avoiding the formation of unwanted tricalcium phosphate and then drying the slurry.

2. A method according to claim l in which a potassium salt is added to the slurry prior to drying.

3. A method of producing a complex fertilizer from natural water insoluble phosphate rock containing phosphorus and calcium which when calculated as P205 and CaO may be represented by the formula P2O5(Ca0)x where x represents the number of mols of CaO in said rock for each mol of P205 present in said rock which method comprises reacting said rock phosphate with sufiicient nitric acid to form a slurry in which all the phosphate is converted to acid form and completely water soluble, then adding suicient ammonia to said slurry to adjust its pH to about 3, then gradually adding to the resulting slurry sulphuric acid in an amount at least equivalent to x-2 mols of H2804 for each mol of P205 present in said slurry and simultaneously while gradually adding said sulphuric acid adding ammonia gradually to said slurry in an amount and at a rate and in a ratio to the sulphuric acid to cause the pH of the slurry gradually to rise to a pH of about 5 while maintaining the temperature of the slurry in a range not substantially higher than C. and not substantially lower than 90 C., and then raising the pH of said slurry to the neighborhood of 5.5 to 6.5 by the addition of ammonia alone, then drying the slurry, thereby to produce a complex fertilizer substantially free from tricalcium phosphate and water aragon to drying.

5. A method of producing a complex fertilizer which comprises forming a slurry by reacting rock phosphate in which the phosphate and calcium present in the `rock Yphosphate vmay be represented by the formula P205(Ca) 'where x equals the number of mols of CaO present for each rnol of P205 present in the rock phosphate, with suficient vnitric acid to :convert all the phosphate to Water soluble phosphoric acid form, then while .maintaining the slurry at a .temperature between 90 C. and 100 C., adding sncient ammonia to raise the pH to about 3 and thereafter adding sulphuric acid Vand ammonia to said slurry at arate -and in proportions gradually to raise the pH of said slurry to about 5, Vthe amount .of sulphuric acid addedbeing the equivalent amount to react :with 4(9c-@Cao for each mol of P205 present in Vthe rock phosphate, then adding Vsuicient ammonia to raise the pH of the slurry to between 5.5 and 6.5 and then drying said slurry, thereby to produce a fertilizer free from water soluble monocalcium phosphate Vand having substantially all of its phosphate .content inra form which is soluble in ammonium citrate. v

6. A method according to .claim 5 in which la potassium salt is .added to the slurry after the Asulphuric acid and ammonia additions.

v `A method `ot .manufacturing a complex fertilizer .in which substantially all of the P205 is present in a form which `is soluble :in ammonium citrate and which compleit-fertilizer is substantially free from Water soluble P205 'and containing nitrogen as N03, nitrogen as NH4 and phosphorus as P205 by the Vattack of natural rock phosphate with sufficient nitric acid kto render all the phosphate in the rock Water soluble and additions of ammonia and `sulphuric acid the said rock being represented -by the formula P2O5(Ca0)1 where x represents the number Vof mols of CaO per mol of P205 which method comprises adding ammonia to the slurry resulting from the reaction of the rock phosphate and nitric acid in such amount as to bring the pH to 3, then gradually adding to said slurry H2804 in an amount equivalent to .ac-2 mols where x represents the number of ymols of Ca to each mol of P205 in the rock phosphate and adding ammonia simultaneously with said H2804 in such proportion as vto gradually increase the pH of the slurry Yuntil it reaches 5 and then adding ammonia alone until .the

slurry does not contain any phosphoric acid in 'water soluble form and meanwhile maintaining the temperature ofthe slurry between v90" and 100 during lthe time the additions are made.

kk8. A method according to claim 7 Yin which a potassium salt is added to the slurry after :the sulphuric acid and ammonia additions.

l9. A continuous process of producing a complex fertilizer containing nitrate nitrogen, ammoniacal nitrogen and phosphate in ,bicalcium lform which comprises continuously vdelivering to a reaction chamber ground phosphate rock together with sutiicient nitric acid to form a `stream of slurry in which the phosphate present in the Vrock is converted to phosphoric acid form soluble in water, continuously delivering said streamof slurry Ythrough a lplurality .of reaction Vvessels Vin series, wherein .ammonia is continuously introduced into the rst and last of said vessels and both ammonia and sulphuric acid are continuously introduced into the intermediate vessels between said -rst and last vessels, ammonia being 8r :continuously introduced into said first vessel ata rate sufiieient onlaintain vthe stream 4of slurry in said' first vessel Iat about pH .3, ammonia .and sulphuric aeid'being simultaneously introduced into Vsaid intermediate vessels at a rate kand in proportion to raise the pH of said stream as said stream of slurry .passes therethrough lto a pH higher than .that ot' :the stream of slurry Vin anyrvesse'ivit has passed through and maintaining the pH of said stream at about y5 when it yenters `said last vessel, continuously introducing ammonia into said `last vessel at a rate -to Y maintain the pH 'of thestrearn of slurry delivered there `from :higher than 5.5 and Athereafter drying the slurry.

1.0.. A method according to claim 9 'm which prior to drying, .a potassium ysait is intermixed with the slurry. l1. A continuous process of producing a complex fertilizer containing nitrate nitrogen, ammoniacal nitrogen and phosphate in bicaicium form vwhich comprises continuously delivering to a react-ion cham-'oer groundY phosphate rock, containing phosphorus and calcium which may be represented by the formula P2O5(CaO),- where x equals the numberof mols of vCaO to each mol of P205 in the rock, together with suicient nitric acid to form a stream .of slurry in which the phosphate present in the rock is converted to phosphoric acid in water soluble form, continuously delivering said stream of slurry through a pluraiity `of reaction vessels in series, wherein ammonia is continuously introduced into the .first and last of said vessels and both ammonia and `sulphuric acid are continuously introduced into the intermediate vessels between said rst and last vessels, ammonia being continuously introduced into said rst vessel lat a rate suicient to maintain the stream of slurry in said rst vessel r,at about pH 3, both ammonia and sulphuric acid being simultaneously introduced into said intermediate vessels at a rate and in .proportion Vto raise the pH .of ysaid stream as said stream of Vslurry passes therethrough to a pH higher than that of the stream of slurry in any vessel it has passed through and maintaining the pH of said stream at about 5 when it enters said last vessel, `the amount of sulphuric acid introduced into said intermediate vessels amounting `at least to x-2 mols of H2804 for each mol of P205 in the slurry passing therethrough, continuously introducing ammonia into said last vessel at a' rate to maintain the stream of slurry delivered therefrom ybetween pH 5.5 and pH A6.5, said stream Aof slurry lat all times being maintained in a temperature range of the .order of to 100 VC. and thereafter drying-:the slurry.

V12. A method according to claim' 11' in which a potassium salt `is added to the slurry prior to drying.

References Cited in the :tile of this patent UNITED STATES PATENTS 1,788,828 Goldberg Ian. 13, 1931 1,948,520 Harvey .,.v Feb. 27, 19,34 2,555,634 VBruijn et al. June 5, 1951 2,555,656 Plusje et al. June 5, 1951 2,611,691 Tramm Sept. 23, 1952 FOREIGN PATENTS 1,644 Great Britain May 19, 1874 340,120 Great Britain Dec. 24, 1930 352,739 Great Britain July 16, 1931 430,380 Great Britain g.. June 18, 1935 OTHER REFERENCES Ammoniated Phosphate, R. S. McBride, -Chem. and Metallurgical Eng., vol. '36, No. 10, Oct. 1929,Y pages 592-595. 

1. A METHOD OF PRODUCING A COMPLEX FERTILIZER FROM WATER INSOLUBE PHOSPHATE ROCK CONTAINING PHOSPHOROUS AND CALCIUM WHICH WHEN CALCUALTED AS P2O5(CAO)X WHERE X REPBE REPRESENTED BY THE FORMULA P205(CAO)X WHERE X REPRESENTS THE NUMERICAL VALUE OF MOLS OF CAO TO EACH MOL OF P2O5 IN SAID ROCK WHICH METHOD COMPRISES REACTING SAID ROCK PHOSPHATE WITH NITRIC ACID IN SUFFICIENT AMOUNT TO FORM A SLURRY IN WHICH ALL THE PHOSPHATE IS CONVERTED TO PHOSPHORIC ACID FORM WHICH IS WATER SOLUBLE, THEN ADDING SUFFICIENT AMMONIA TO SAID SLURRY TO ADJUST ITS PH TO ABOUT 3, THEN GRADUALLY ADDING TO THE RESULTING SLURRY SULPHURIC ACID IN AN AMOUNT EQUIVALENT TO X-2 MOLS OF H2SO4 FOR EACH MOL OF P205 PRESENT IN THE SLURRY AND SIMULTANEOUSLY WHILE ADDING SAID SULPHURIC ACID ALSO ADDING AMMONIA GRADUALLY IN AN AMOUNT AND AT A RATE AND IN A RATIO TO THE SULPHURIC ACID TO CAUSE THE PH OF THE SLURRY GRADUALLY TO RISE TO A PH OF ABOUT 5, AND THEN ADDING SUFFICIENT AMMONIA ALONE TO THE SLURRY TO RAISE THE PH OF THE SLURRY TO THE NEIGHBORHOOD OF 5.5 TO 6.5 THEREBY TO CONVERT ANY EXISTING MONOCALCIUM PHOSPHATE TO DICALCIUM PHOSPHATE AND AVOIDING THE FORMATION OF UNWANTED TRICALCIUM PHOSPHATE AND THEN DRYING THE SLURRY. 